1. Field of the Invention
This invention relates to computer graphics, and more specifically to a computer system that generates graphic images of variable image complexity at a constant frame rate. The invention uses reduction and magnification capabilities to generate graphic images at a constant frame rate, thereby creating a continuous and smooth visual effect.
2. Related Art
Many computer applications that use computer graphics require the system to generate those graphic images at a constant frame rate. These applications include visual simulation and virtual reality in which the user operates in an interactive graphic environment. Specific environments include: pilot and driver training, medical and surgical preview, medical diagnosis and analysis, computer-aided design, and any other application where a user needs to walk-through or manipulate data. In these applications, the graphic images are the most important feature of the system. Users require the images to be continuous with smooth transitions as they move their heads or fields of vision.
Continuous graphic images are ensured when the computer system generates those images at a constant frame rate. The system must process and generate each image within the time limit of one frame or field of video. The system cannot extend the processing of an image beyond the time limit of one frame without impacting the visual effect and generating a variable frame rate. When a system does not adhere to a constant frame rate, and generates images at a variable frame rate, the user is subject to sudden and abrupt image updates. Such spasmodic updates detract from the realism of the experience and are visually disturbing to the user. Therefore, a constant frame rate is required to achieve a realistic and effective interactive graphic environment.
In addition to the user's need for a constant frame rate, the majority of display devices used with interactive graphic environments only accept graphic images for display at a constant frame rate. These devices cannot handle the receipt of graphic images at a varying rate. These display devices include computer monitors, projectors, television sets, video cassette recorders (VCR), and video display helmets. Similarly, all of these devices display the graphic images at a constant output video rate. The frame rate at which the computer system generates the graphic images may, or may not, be equivalent to the video rate of the display device.
During a simulation or other interactive graphic environment, there are varying computational burdens placed on the video hardware of a computer system. This computational burden on the system varies depending on the image complexity. The burden increases in proportion to the increase of image complexity. The graphical image to be generated and displayed may range from a simple view of the sky to the complex view of a city. In each instance, the video hardware must generate the image within the time allotted for a single frame.
The problem arises when the image complexity prevents the graphics hardware from drawing the image within the time limit of one frame. If the processing time of an image extends beyond one frame, the typical result is that the image currently displayed is repeatedly displayed for as many frames as it takes the video hardware to finish the computation of the new image. This repetitious display of the same image reduces the actual video display rate and seriously compromises the realism of a simulation.
One approach to ensuring a constant frame rate is to set the frame rate low enough to ensure that an image of any complexity could be generated and displayed within one frame. This is an unreasonable solution for several reasons. First, the video hardware would always be executing at its lowest capacity. Second, the video effect on the user would be degraded and detract from the realism of the experience.
A second approach to maintaining a constant frame rate focuses on only drawing objects of the image that are in the field of view, and reducing the complexity of the objects that are far away. This approach maximizes the performance of the graphics hardware, as opposed to guaranteeing a specific frame rate.
A third approach involves the use of vector displays which are monitors that can display images at a variable frame rate. Thus, if it takes longer than the nominal frame time to draw an image, the frame time is simply extended. This approach has two disadvantages. First, vector displays are expensive compared to conventional raster monitors. Second, the frame time can only be extended slightly before either a visible flicker or image smear occurs when displaying the image, thereby impairing the realism of the display.
Therefore, there is a need for a computer system to automatically adjust to the complexity of an image on a per frame basis when generating graphic images at a constant frame rate.